Shards of Slagmite

Creatures you control have haste.
At the beginning of your first postcombat main phase each turn, add {r} to your mana pool for each tapped basic Mountain you control.
"It seems alive, reacting to whatever spellcaster or artifact draws near. The stronger the spell, the greater the reaction, and the greater the risk. It is
to be noted that slagmite can be quite... sharp."
-Grenka Jyn